In general, frequency adjustment in an electric power system and adjustment of power demands and power supplies in the electric power system are carried out using a plurality of power generators, storage batteries, etc., equipped in the electric power system. Further, in most cases, adjustment in the difference between the generated electric power from natural energy based power generators and its planned output electric power, and reduction in the changes of electric power generated by the natural energy based power generators are also performed using the power generators, storage batteries, etc. In comparison with general electric power generators, the storage batteries can change the electric power output at high speed, and can be used effectively in frequency adjustment of the electric power system, adjustment of the difference between the generated electric power from natural energy based power generators and its planned output electric power, and adjustment of power demands and power supplies in the electric power system.
In this regard, as a storage battery operated at high temperature connected to the electric power system, a sodium-sulfur battery (hereinafter referred to as the NaS battery) is used, for example. This NaS battery is a high temperature secondary battery containing metal sodium and sulfur as active materials in an isolated manner using a solid electrolyte tube. When the NaS battery is heated at high temperature of about 300° C., a certain amount of energy is produced by an electrochemical reaction of both of the melted active materials of these metal sodium and sulfur. Normally, the NaS battery is formed by assembling a plurality of battery cells upright, and used in a form of a module including a plurality of battery cells connected together. That is, the module has structure where circuits (strings) each formed by connecting a plurality of battery cells in series are connected in parallel to form a block, and at least two blocks are connected in series, and placed in a heat insulating container.
As a method of detecting a failure in the module of this type, a method of detecting a failure in a battery by comparing the depth of discharge in each block is disclosed (e.g., see Japanese Laid-Open Patent Publication No. 03-158781). In this method, the presence of the failure in the battery is considered for each of the blocks of the module. Therefore, in comparison with the method of detecting a failure in each of NaS battery cells forming the block, the apparatus is not complicated, and the production cost is reduced. In these respects, this failure detection method is advantageous.